The present invention relates to electronic devices, and, more particularly, to low-power data communication over a serial channel.
Battery-powered and portable electronic modules and systems have found increasing applications due to advantages such as: portability; improved immunity to electrical noise; nonvolatile memory; improved safety; lighter weight; improved capaciltiy for international marketing; and simplified regulatory requirements. Battery-powered systems demand very low power for long battery lifetime and small battery size.
However, serial data communication with conventional protocols implemented in a straightforward fashion consumes significant power and would quickly drain a battery.
For example, the RS232 standard, widely used in microcomputer systems, specifies a 3 to 7 .OMEGA. load resistor connected to ground, and the RS232 data line will normally remain in the mark state (typically at -12 volts) when idle and alternate between marks and spaces (typically at +12 volts). Thus a substantial current must be sourced when transmitting with a straightforward implementation, and if the data lines remain connected after a communications session has ended, power will statically flow into the transmitters and drain a battery. Despite this power consumption, the RS232 has a number of advantages: it is already widely used, the connections are simple (only a receive line RX, a transmit line TX, and ground line), and the standards are actually somewhat flexible in practice. Thus efforts have been made to design RS232 interfaces that extend performance; see for example, U.S. Pat. No. 4,607,170 and the Cross-Referenced Applications.
Battery-powered systems frequently must support both RS-232 and TTL interfaces, but it is a problem with the known interfaces to distinguish between RS232 and TTL operation.
The present invention provides a transceiver with automatic operation for both RS232 and TTL interfaces by sensing of the received voltage levels to determine the mode of operation, and senses the negative levels of RS232 inputs to steal current with surge protection to provide output transmitter power, and includes hysteresis on the negative level sensing and provides selectable output levels, slew rates, polarities together with selectable input impedances and polarities.